enhance(pull)!: add per-repository default squash merge commit message (#39816)

Adds a repository setting for the default squash merge commit message
body with GitHub's options `PR_BODY`, `COMMIT_MESSAGES` and `BLANK`,
also settable via `squash_merge_commit_message` in the API. The commit
title stays the pull request title.

1. Breaking: API squash merges without `merge_message_field` now get the
same default message as the web merge box instead of a title-only
commit, which affects clients sending only `Do` like Renovate. An empty
value keeps title-only.
2. Breaking: `[repository.pull-request] DEFAULT_SQUASH_MERGE_MESSAGE`
replaces `POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES`, which still
works as a deprecated fallback. Repositories store the default when
created or next saved.
3. Deprecated app.ini keys now migrate through one table instead of
per-loader fallbacks, which also fixes the ignored `[mailer] CERT_FILE`
and `KEY_FILE` fallbacks. Deprecation warnings no longer claim a removal
version.

<img width="1891" height="571" alt="grafik"
src="https://github.com/user-attachments/assets/0fdcde13-1f11-4156-b598-d80eb0d04cbe"
/>

---------

Co-authored-by: silverwind <me@silverwind.io>

custom/conf/app.example.ini

@@ -1218,8 +1218,9 @@ LEVEL = Info
 ;; In default merge messages only include approvers who are official
 ;DEFAULT_MERGE_MESSAGE_OFFICIAL_APPROVERS_ONLY = true
 ;;
-;; In default squash-merge messages include the commit message of all commits comprising the pull request.
-;POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES = false
+;; Set default squash merge commit message body, valid options: PR_BODY (pull request description), COMMIT_MESSAGES (commit messages), BLANK (none)
+;; Repositories store the current value when created or when their settings are next saved, and can change it there.
+;DEFAULT_SQUASH_MERGE_MESSAGE = PR_BODY
 ;;
 ;; Add co-authored-by and co-committed-by trailers if committer does not match author
 ;ADD_CO_COMMITTER_TRAILERS = true